The recent ratification of IEEE 802.15.4 PHY-MAC specifications
for low-rate wireless personal area networks represents a
significant milestone in promoting deployment of wireless sensor
networks (WSNs) for a variety of commercial uses. The 15.4
specifications specifically target wireless networking among
low rate, low power and low cost} devices that is expected to be a
key market segment for a large number of WSN applications. In this
paper, we first analyze the performance of the contention access
period specified in IEEE 802.15.4 standard, in terms of throughput
and energy consumption. This analysis is facilitated by a modeling
of the contention access period as non-persistent CSMA with
backoff. We show that in certain applications, in which having an
inactive period in the superframe may not be desirable due to
delay constraints, shutting down the radio between transmissions
provides significant savings in power without significantly
compromising the throughput. We also propose and analyze the
performance of a modification to the specification which could be
used for applications in which MAC-level acknowledgements are not
used. Extensive ns-2 simulations are used to verify the
analysis.